Behavior of joining mapresults

[Christian Köstlin]

When working with json data files, that we're a little bigger than
convenient I stumbled upon a strange behavior with joining of mapresults
(I understand that this is more or less flatmap).
I mapped inputfiles, to JSONValues, from which I took out some arrays,
whose content I wanted to join.
Although the joiner is at the end of the functional pipe, it led to
calling of the parsing code twice.
I tried to reduce the problem:

#!/usr/bin/env rdmd -unittest
unittest {
    import std.stdio;
    import std.range;
    import std.algorithm;
    import std.string;

    auto parse(int i) {
        writeln("parsing %s".format(i));
        return [1, 2, 3];
    }

    writeln(iota(1, 5).map!(parse));
    writeln("-------------------------------");
    writeln((iota(1, 5).map!(parse)).joiner);
}

void main() {}

As you can see if you run this code, parsing 1,..5 is called two times
each. What am I doing wrong here?

Thanks in advance,
Christian